1. Field of the Invention
The present invention relates to an electrically controlled, pressurized fluid powered transmission shift control system. In particular, the present invention relates to an electrically controlled, fluid actuated shift control systems of the type having at least one multiple position actuator selectively positioned by one or more electrically controlled two position valves having either a normally open or normally closed condition, one position of said actuator corresponding to all of said valves being in the normal positions thereof. More particularly, the present invention relates to an electrically controlled fluid pressure actuated shift control system having a mechanism for retaining the actuator in the selecting position thereof in the event of an interruption or failure of the source of electrical power or of pressurized fluid.
2. Description of the Invention
Automatic or semi-automatic change gear transmissions including fluid power actuators controlled by electrical command output signals from a system controller, usually a microprocessor based ECU, are well known in the prior art as may be seen by reference to U.S. Pat. Nos. 4,361,060; 4,648,290; 4,595,986; 4,873,881; and 4,722,248, the disclosures of which are hereby incorporated by reference.
Fluid pressure actuated, electrically controlled transmission shift actuators, of both the multiple parallel piston type and the X-Y shifter type are well known in the prior art as may be seen by reference to U.S. Pat. Nos. 4,928,544; 4,748,863; 4,445,393 and 4,046,032, the disclosures of which are hereby incorporated by reference.
Typically, such prior art control systems comprised a plurality of two, three or four position actuators having a plurality of complimentary and/or opposed pistons, often differential area pistons, the pressurization or exhausting of which pistons being controlled by a plurality of electrically controlled, usually solenoid controlled, two position valves biased to a normally open or a normally closed condition and, upon energization of the solenoid, displaceable to the other position thereof.
While the prior art electrically controlled, pressurized fluid powered shift actuators for drive train components, such as change gear transmissions, were well received, they were not totally satisfactory as, in the event of an interruption or failure in the electrical power supply, the valves will revert to the normally open or normally closed positions thereof and the actuator will be caused; possibly undesirably, to assume the default position thereof, often a neutral or disengaged position. Assuming such a default position may be undesired as the vehicle may lose the limp home/limp off road possibility and/or engine braking may be lost.